There is known a pulley construction (cf. SU, A, 707,880) comprising a hub with a rim, the rim base accommodating a movable lining in the form of a closed shaped strip. In the course of operation the rope moves together with the lining relative to the pulley rim, rather than relative to the lining. Therewith, the corrugated surface of the rope virtually fails to wear the lining. The lining is subject to slow wear at the side of the smooth surface of the rim under specific pressures of substantially smaller magnitude than those taking place with the fixed lining caused by ropes in the conventional prior art constructions of rope guiding pulleys.
Arrangement of the movable lining in the form of a shaped strip allows to make a fast replacement of the worn lining without removing the rope from the pulley. This in turn makes way to a wider application of such a movable lining in pulley blocks of drilling rigs. The procedure of removing the worn lining from the pulley rim and fitting a new lining to the rim can be done by a slowly moving rope. The work of the operator in this case resides in detaching the ends of the worn movable lining, placing a new lining under the rope at the rope climbing side, and closing the ends of the new lining into a ring around the rim after the rope pulls the lining into the rim base.
However, high rope stress between the pulley rim and shaped strip causes intensive wear of the lining necessitating stops of hoisting machines for lining replacement.
In order to ensure that the movable lining is more reliable, the shaped strip (cf., SU, A, 783,210) is made of a rubberized band having portions impregnated with epoxy resin at the side of contact between the rope and pulley rim and alternating non-impregnated portions.
However, this movable lining in the form of a rubberized band embracing the pulley about the rim circumference is subject to tensile stresses caused by centrifugal forces outside the arc of contact between the pulley and rope and tensile stresses arising due to forces of friction caused by resilient stretching of the rope at the arc of contact. The later forces can be substantial due to high cohesion between the lining and ropes, whereby the lining repeats oscillating motions of the rope in the course of resilient stretching of an amplitude 0.5 to 3 mm. Tensile stresses in the rope and deformations associated therewith can reach critical magnitudes for the lining to result in inadmissibly high residual deformations. The residual deformation gradually builds up to an extent causing escape of the lining from under the rope and failure.
The lining, as well as the rope, is stationary relative to peripheral sections of the arc of contact, although it has a tendency to move at the points where it climbs and runs off the pulley. In consequence, one portion of the lining is clamped, whereas the other two oscillate together with the rope alternately elongating and contracting thereby experiencing tensile stresses especially dangerous as the cross section of the movable lining is reduced due to wear. Therewith, the sign-variable oscillating character of stresses caused by resilient stretching of the rope can give rise to fatigue in the lining, again hazardous as its cross section is reduced in the course of wear.
Therefore, low reliability of the movable lining in this known pulley is caused by that this prior art construction is incapable of reliably performing two major functions, viz., prevent the rope and pulley rim from wear and act as a carrying element holding the lining in the pulley rim.
There is also known a lining in which lining inserts are freely placed onto a flexible link, clearances between the inserts accommodating prestressed resilient elements embracing with a clearance the flexible link (cf., SU, A, 1,081,949).
However, these inserts fail in the course of prolonged operation due to alternating dynamic loads. Replacement of the lining inserts requires removal of the movable lining from the pulley which is time consuming and leads to idling of hoisting machines in mines.